1. Technical Field
The present invention relates to natural gas liquid (NGL) processes. More particularly, this invention relates to reducing the amount of carbon dioxide (CO2) recovered with NGL during cryogenic processing.
2. Description of Prior Art
Natural gas and refinery off gas streams generally contain more volatile components such as hydrogen, methane, carbon monoxide, CO2, nitrogen and heavier hydrocarbon components such as ethane, ethylene, propane, propylene, and other heavier components. The amount of these components present depends on the source of the feed gas. Recovery of ethane and ethylene from natural gas and refinery off gas streams is a common hydrocarbon recovery process. However, along with the ethane and ethylene, a significant amount of the more volatile components, such as CO2, in the feed stream will also be recovered with NGL in this process. Pipelines generally have a maximum allowable amount of CO2 that is permissible in NGL. As a result of this, recovered CO2 may need to be removed with downstream equipment to meet the CO2 specification limits in the NGL. The additional equipment required to remove the CO2 adds considerable capital and operating costs to the process.
In order to reduce the amount of CO2 contained in the NGL stream, CO2 needs to be extracted from the NGL stream by treating it with an amine. Once the CO2 is removed, it is typically vented to the atmosphere. The amine system needed to treat the NGL stream will need a significant amount of fuel to regenerate itself, which sends even more CO2 to be vented to the atmosphere. If the NGL recovery plant is producing liquid hydrocarbon that is to be used by a petrochemical plant, the ethane from the NGL is fractionated out and treated for CO2 removal. Again, the treating is done by amines, and leads to significant excess CO2 venting to atmosphere.
As an alternative method of CO2 reduction, the feed gas can be treated to reduce the amount of CO2 in the feedstream, which will in turn reduce the amount that is recovered with the NGL during cryogenic processing. However, pretreating the feed gas stream also adds considerable costs to the overall NGL process.
In many NGL recovery processes, there is little control over the amount of CO2 that is recovered with the NGL. If higher C2 recovery is desired, NGL will contain more CO2. In order to reduce the amount of CO2 in NGL, the fractionation tower used in the process needs to be reboiled more. The increased reboiler activity in turn will lead to some loss of desirable components, such as ethane and ethylene, or a loss of process efficiency if the same recovery is maintained.
In a typical turbo expander plant, feed gas is treated to remove impurities such as water, mercury, etc. and then sent for hydrocarbon recovery. If the feed gas pressure is not high enough, compression of the feed gas may be utilized. Gas entering the cryogenic section of the plant is first cooled in one or more exchangers to at least partially condense the gas. The two-phase stream is then sent to a cold separator to separate the vapor from the liquid. For an ethane and heavier compound (xe2x80x9cC2+xe2x80x9d) recovery process, the liquid stream is expanded and sent to a fractionation tower, while the vapor stream is expanded with a work expansion device, such as a turbo expander, and sent to the fractionation tower as an upper tower feed stream. A bottom reboiler is provided for the fractionation tower to control the amount of lighter components exiting the bottom of the fractionation tower with desirable C2+ components. One or more side reboilers are added to the fractionation tower to increase efficiency of cross exchange. The overhead of the fractionation tower is the cold residue gas, which essentially contains the lighter components in the feed gas. Residue stream is preheated in the cross exchanger train and then sent for further processing. Further processing could include compression and cooling of the gas to the desired pressure and temperature.
For a high C2 recovery process, a reflux stream is required above the expander outlet feed location in the fractionation tower to recover some of the C2+ components that are leaving the top of the tower. Several sources for a reflux stream can be used. One source can be at least a portion of the warmed and compressed residue gas. A part of this high-pressure residue gas is cooled in the chilling train and substantially condensed. This condensed stream, which is lean in C2+ components, is fed above the expander outlet feed of the fractionation tower. Such a process is able to recover well in excess of 95% of the C2+ components. An alternate source of a reflux stream can be at least a portion of the vapor stream being sent to the expander. This stream is condensed under pressure and sent as a top feed stream to the fractionation tower. Such a process can produce C2+ recovery in the low to middle 90""s %. Yet another source for a reflux stream is to take at least a portion of the expander feed gas and partially condense it. This condensed stream is sent at a lower location in the fractionation tower, while the vapor stream that is leaner in C2+ components than the expander feed stream is condensed under pressure and sent as top feed to the fractionation tower. Such a process can produce C2 recovery in the middle 90""s %.
Several new processes have been developed in recent years that use multiple reflux streams above the expander outlet feed location in the fractionation tower. These processes generate streams of various C2+ richness levels and use them at different locations in the fractionation tower to increase ethane recovery and efficiency of the process. These multiple reflux processes are capable of C2 recovery well in excess of 95%.
Not only is recovery of NGL an issue, but the removal of other components from either the NGL stream or the residue gas is also important. An example process in which CO2 is removed from the residue gas stream can be found in U.S. Pat. No. 5,960,644 issued to Nagelvoort et al. In Nagelvoort, natural gas is condensed and then separated into a liquid stream and a vapor stream. The vapor stream is sent to a fractionation tower and the liquid stream is also sent to the tower below the vapor stream. A stream taken from the tower, reboiled, and returned to the tower at location below the liquid stream feed location. The tower produces an overhead stream, which is condensed and separated. The resulting liquid stream is refluxed back to the tower at a higher location than the vapor stream feed location. The resulting vapor is condensed and separated again. The resulting liquid stream is refluxed back to the tower as a second reflux stream at a higher location than the first reflux stream. This process removes the CO2 from vapors and refluxes the CO2 back into the column. Ultimately, the tower bottoms liquid stream contains the majority of CO2, which has to be removed with further processing, and the residue gas stream is relatively free of CO2.
Others have developed processes to try to reduce the amount of CO2 contained within the NGL liquids that are recovered from natural gas streams. An example can be found in U.S. Pat. No. 4,185,978 issued to McGalliard et al. In this process, a hydrocarbon feed gas is expanded, separated, and sent to a demethanizer tower. The demethanizer tower produces an overhead stream containing essentially all of the methane and gaseous CO2 and a bottoms stream containing essentially all of the liquid ethane and heavier components, along with non-gaseous CO2 dissolved in the liquid stream. To remove the CO2 from the liquid stream, an external inert sweep gas is injected into the liquid stream as a stripping gas. This stripping gas helps regulate the reboiler temperatures to reduce temperature fluctuations within the tower that can lead to significant swings in the amount of CO2 that is recovered in the NGL liquid streams.
A need exists for a more economical and efficient method of reducing the amount of CO2 that is recovered in the NGL cryogenic processes. A need also exists for a process to NGL streams with reduced amounts of CO2 in the NGL stream, as opposed to processing the stream further to remove CO2. A further need exists for a method of reducing CO2 in NGL streams without having to add additional chemicals, which increases the operating costs of the process. It is an object and goal to provide a process and apparatus to reduce the amount of CO2 recovered in the NGL product. It is an additional object and goal to improve ethane recovery in the NGL product when CO2 recovery is maintained.
The present invention includes a process and apparatus for reducing the amount of CO2 that is recovered in a NGL product stream. The invention can also be used to increase the amount of ethane and ethylene recovery in the NGL product stream, while maintaining the same amount of CO2 in the NGL product stream. In this process, a cold separator is used to separate the feed into a first liquid stream and a first vapor stream. The first liquid stream is then divided into two streams, a second liquid stream and a third liquid stream. The third liquid stream is heated and supplied to a fractionation tower as a stripping gas at a point below the other feed streams. The stripping gas strips the CO2 from the liquids falling down the tower. The result of this stripping mechanism is reduced CO2 in the NGL product stream or increased ethane and ethylene recovery with maintained CO2 recovery levels.
The present invention is applicable for the separation of ethane, ethylene, propane propylene and other C3 components and heavier components from the above mentioned feed gases using cryogenic turbo expander process. The present invention can be modified to use two separate towers, an absorber tower and a fractionation tower. Other variations can be used, such as a split vapor feed stream and using a portion of a residue gas stream as a reflux stream in the fractionation tower.
The apparatus preferably includes an inlet heat exchanger, an expander, a fractionation tower, at least one side reboiler, and a splitter for splitting the first liquid stream to provide a stripping gas for the fractionation tower. An absorber tower can also be used, as described herein.